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Collision welding is a promising approach for joining conventional materials in identical or dissimilar combinations without heat-related strength loss, thereby opening up new lightweight potential. Widespread application of this technology is still limited by an insufficient state of knowledge with respect to the underlying joining mechanisms. This paper applies collision welding to a material combination of DC04 steel and EN AW 6016 aluminium alloy. Firstly, the welding process window for the combination is determined by varying the collision speed and the collision angle, the two main influencing variables in collision welding, using a special model test rig. The process window area with the highest shear tensile strength of the welded joint is then determined using shear tensile tests and SEM images of the weld zone. The SEM investigations reveal four distinct metallographic structures in the weld zones, the area fractions of which are determined and correlated with collision angle and shear tensile strength. [ABSTRACT FROM AUTHOR]

In this study, a novel embedded machine learning (ML) framework was employed to analyze interfacial characteristics in dissimilar-alloy impact welding. While keeping the substrates constant, i.e., stainless steel and Ti alloy, the influence of interlayer types on the resultant morphology of impact-welded bonds was also highlighted. The dataset was meticulously generated through numerical simulations, utilizing materials properties and processing parameters as primary inputs. These inputs were used to predict various interfacial features, including the extent of wave formation (L), specific welding zone area (S), average waveform amplitude (A), and maximum interface temperature (Tmax). Impressively, exceptional predictive capabilities were demonstrated by the ML model, with determination coefficients of 0.938, 0.924, 0.967, and 0.909 for S, L, A, and Tmax, respectively. Based on the results, strong dependencies between interfacial features and the weight functions of materials properties and processing parameters were unveiled. Intriguingly, as the weight function of materials properties for a specific output objective increased, a corresponding decrease in the weight function of processing parameter inputs was observed. These findings emphasize the complex connection between input parameters and output characteristics, revealing previously unexplored intricacies in the impact welding process, facilitated by our advanced ML methodology. [ABSTRACT FROM AUTHOR]

This study demonstrates that the thickness of the target and its backing condition have a powerful effect on the development of a wave structure in impact welds. Conventional theories and experiments related to impact welds show that the impact angle and speed of the flyer have a controlling influence on the development of wave structure and jetting. These results imply that control of reflected stress waves can be effectively used to optimize welding conditions and expand the range of acceptable collision angle and speed for good welding. Impact welding and laser impact welding are a class of processes that can create solid-state welds, permitting the formation of strong and tough welds without the creation of significant heat affected zones, and can avoid the gross formation of intermetallic in dissimilar metal pairs. This study examined small-scale impact using a consistent launch condition for a 127 µm commercially pure titanium flyer impacted against commercially pure copper target with thicknesses between 127 µm and 1000 µm. Steel and acrylic backing layers were placed behind the target to change wave reflection characteristics. The launch conditions produced normal collision at about 900 m/s at the weld center, with decreasing impact speed and increasing angle moving toward the outer perimeter. The target thickness had a large effect on wave morphology, with the wave amplitude increasing with target thickness in both cases, peaking when target thickness is about twice flyer thickness, and then falling. The acrylic backing showed a consistently smaller unwelded central zone, indicating that impact welding is possible at a smaller angle in that case. Strength was measured in destructive tensile testing. Failure was controlled by the breakdown of the weaker of the two base metals over all thicknesses and backings. This demonstrates that laser impact welding is a robust method for joining dissimilar metals over a range of thicknesses. [ABSTRACT FROM AUTHOR]

This paper focuses on the nanoscale characterization of a welded Al/Cu interface produced by magnetic pulse welding. Welding tests were performed using three field shapers with similar dimension but having strongly different electrical conductivity. Other welding parameters were strictly the same. The three tests produce similar welded joints that consist of a similar morphology of wavy interface at the macroscopic scale. Thus, the Al to Cu transition zone across the location of an interfacial wave is identified as a suitable repetitive site that enables for repetitive fine-scale characterization across a distance less than 1 µm. Transmission Electron Microscopy (TEM) observations of the welded Al/Cu interface reveal a gradient of nanostructures which consists of an amorphous AlxCuy nanolayer (∼30 nm) and then, a nanocrystalline layer with a thin thickness of a few tens of nanometres at the Cu side and at the Al side as well. This hierarchical nano-featured structure is confined within a very short total distance of about 200 nm. Outside these confined nanostructures, the interface exhibits a crystalline structure. These nanofeatures were observed at the interface of the Al/Cu welded joint produced by three different field shapers, that shows a reproducibility of the interface structure at the nanoscale level. A thermomechanical analysis of the high strain process at a collision point allows for depicting the mechanism of microstructure formation confined at the Al/Cu interface. The shear strain rate across the interface shows peaks where the temperature distribution also reaches a peak beyond the melting point of Al. The thermal kinetics within the molten zone is characterized by a high cooling rate up to 103 °C/ns during the melting/solidification stage that explains the observation of the amorphous nanolayer revealed by the TEM observation. The formation of nanocrystalline structure confined at both sides of the amorphous layer can be explained by a nucleation of crystals at a very early stage governed by the thermal kinetics at the boundaries (Al side and Cu side) of the melted zone, and by a dynamic recrystallization governed by the gradient of shearing at high strain rate confined at the collision point. Together, these processes of highly transient thermomechanical responses confined at the Al/Cu interface explain the formation mechanism of interface microstructure that results in the hierarchical nanostructure as identified by TEM characterizations.

Collision welding is a promising approach for joining conventional materials in identical or dissimilar combinations without heat-related strength loss, thereby opening up new lightweight potential. Widespread application of this technology is still limited by an insufficient state of knowledge with respect to the underlying joining mechanisms. This paper applies collision welding to a material combination of DC04 steel and EN AW 6016 aluminium alloy. Firstly, the welding process window for the combination is determined by varying the collision speed and the collision angle, the two main influencing variables in collision welding, using a special model test rig. The process window area with the highest shear tensile strength of the welded joint is then determined using shear tensile tests and SEM images of the weld zone. The SEM investigations reveal four distinct metallographic structures in the weld zones, the area fractions of which are determined and correlated with collision angle and shear tensile strength.

This paper investigates the transformation of Mg-Si precipitates formed at an interface of an Al-Mg-Si alloy joint, welded by the high strain-rate collision during magnetic pulse impact welding. The Transmission Electron Microscopy (TEM) and Energy dispersive X-Ray (EDX) analysis of the precipitates reveal a nucleation of circular shape AlFeMnSiCr precipitate (size of ∼100 nm) from the dissolution of the native β″ (needle-shaped) precipitate. The AlFeMnSiCr phase can also reach a size up to several microns and grows into diverse shapes (rod-shaped, polygonal-shaped, ovoid-shaped). Based on the microstructural characterizations and on observations available in the literature including relevant data of Lodgaard and Ryum, the transformation sequence due to the high-speed collision can be described as follows: β″-Mg-Si (needle-shaped) → β’-Mg2Si (rod-shaped) ‘u-phase’ → Al(MnCrFe)Si (round-shaped) → Al(MnCrFe)Si +(+: Cr; Cu and/or Ni, rod-shaped, polygonal-shaped, ovoid-shaped). The comparison of samples welded at three different impact conditions (low impact intensity, intermediate impact intensity, high impact intensity) shows that the area fraction of the precipitates decreases with the increase of the impact intensity. The identification of the crystal structure of these different precipitates, from the nucleation to the final complete formation of the precipitation, can further enrich the transformation sequence in this paper. The role of the high strain kinematics on the deformation of the Al lattice and on the diffusion rate of alloying elements is also a perspective that can further explain this new observation of precipitation during magnetic pulse welding.

This study demonstrates that the thickness of the target and its backing condition have a powerful effect on the development of a wave structure in impact welds. Conventional theories and experiments related to impact welds show that the impact angle and speed of the flyer have a controlling influence on the development of wave structure and jetting. These results imply that control of reflected stress waves can be effectively used to optimize welding conditions and expand the range of acceptable collision angle and speed for good welding. Impact welding and laser impact welding are a class of processes that can create solid-state welds, permitting the formation of strong and tough welds without the creation of significant heat affected zones, and can avoid the gross formation of intermetallic in dissimilar metal pairs. This study examined small-scale impact using a consistent launch condition for a 127 µm commercially pure titanium flyer impacted against commercially pure copper target with thicknesses between 127 µm and 1000 µm. Steel and acrylic backing layers were placed behind the target to change wave reflection characteristics. The launch conditions produced normal collision at about 900 m/s at the weld center, with decreasing impact speed and increasing angle moving toward the outer perimeter. The target thickness had a large effect on wave morphology, with the wave amplitude increasing with target thickness in both cases, peaking when target thickness is about twice flyer thickness, and then falling. The acrylic backing showed a consistently smaller unwelded central zone, indicating that impact welding is possible at a smaller angle in that case. Strength was measured in destructive tensile testing. Failure was controlled by the breakdown of the weaker of the two base metals over all thicknesses and backings. This demonstrates that laser impact welding is a robust method for joining dissimilar metals over a range of thicknesses.

Background. Electromagnetic Pulse Technology (TEM) is a non-contact pulse process with great potential for connecting dissimilar materials. This method uses pulsed electromagnetic fields to form and connect metals with high conductivity, such as aluminum or copper. This process makes it possible to perform connections with geometric closure and tension due to compression or expansion of profile structures of alloys with a closed cross section, as well as metallurgical welds. Despite the technological advantages and potential, only a modest introduction into industrial applications has been found. This scientific article aims to reveal the industrial interest and applicability of electromagnetic pulse technology in various subsectors of the microelectronic industry, promoting lightweight and highprecision engineering components. Materials and methods. Based on numerical and experimental studies, copper-copper and copper-aluminum electrical connections are produced by electromagnetic pulse crimping. The main parameters are determined and their influence on the connection process and the achievable strength of the connection is analyzed. Attention is paid to the development of electromagnetic drives. The fundamentals of the theory of magnetic and pulsed welding are applied. Results. Conclusions and instructions on the design of the manufacture of compounds using electromagnetic pulse technology are given. A simplified approach to modeling is presented for numerical studies. This unrelated approach makes it possible to successfully develop the process and design of the connection for electromagnetic pulse crimping. Various welding parameters are indicated, which determine the different morphology of the surface, the section with different mechanical strength. The influence of the mandrel surface on the quality of the joint is evaluated. Conclusions. The weldability criteria for copper and aluminum compounds have been experimentally determined. The influence of several parameters of the experimental welding process, namely the discharge energy, gaps and holes, is evaluated using destructive tests.

A technique for producing impact welds with small standoff gaps between dissimilar aluminum alloys, using the Vaporizing Foil Actuator Welding technique, is demonstrated here with the joining of aluminum 6061-T6 to aluminum 7075-T6. One-millimeter-thick AA6061-T6 flyers were welded to a 3-mm thick 7075-T6 target with a nominal standoff gap of just 0.3mm. This small standoff gap is an improvement compared to other impact welding work, allowing for less deformation in the flyer sheet. The welds exhibited remarkable consistency in their failure loads during tensile tests, with nugget pullout mode failure in all samples at failure loads similar to or exceeding those of a comparably sized rivet. Cyclic testing was not as repeatable as the static testing. While the method adopted here requires bare metal surfaces for welding, the process can make joints without damage to nearby coated surfaces. A geometric model is proposed to help explain how sufficient impact angles are generated over standoff distances of 0.3mm or less. The model agrees well with the morphology of the welded and unwelded areas produced in this work and may be useful for predicting impact angles as a function of the standoff distance and the shape of the foil actuator used to launch the flyer to high impact speeds. [ABSTRACT FROM AUTHOR]

While high velocity impact welding is perceived as a solid-state joining process, localized or continuous melting on the weld interfaces has posed a significant challenge. Here we propose to overcome this challenge by introducing a nanocrystalline layer on the metal surface for impact welding of magnesium to aluminum. After surface nanocrystallization, melting and intermetallic (IMC) formation at the interface were suppressed, resulting in a 60% improvement in Mg-Al joint strength. This is because the surface nanocrystalline layer limited the plastic shearing deformation and deformation heat at the Mg-Al interface during impact welding. Meanwhile, the high density of grain boundaries in nanocrystalline Mg layer and the high density of dislocations inside the nanocrystalline provided short-circuiting paths for Al diffusion into Mg matrix, forming a diffusion layer of 30–45 nm thickness. This work provides a new approach to dramatically improve dissimilar impact welding strength via surface nanocrysallization. [Display omitted] [ABSTRACT FROM AUTHOR]

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To explore the optimal method for simulating impact welding, we compared detailedly the smoothed particle hydrodynamics (SPH) and structured arbitrary Lagrangian-Eulerian (S-ALE) methods in predicting bonding interface. Based on the experimental results of Cu-Q235 steel atmospheric explosive welding, an in-depth analysis was presented. The results show that the SPH and S-ALE methods converged at element sizes of 2.5 and 2 μm, respectively. When the element size was further reduced to 0.5 % of the wavelength, the errors of both methods were less than 5 %. Benefiting from the gradient mesh, the efficiency of the S-ALE method was 2–4 times that of the SPH method at the same element size. With the addition of the gas medium, the S-ALE method derived the formation mechanism of the gap gas shockwave and indicated that the pressure distribution is not uniform. Furthermore, the local high pressure of the gas shockwave prevents the vortex closure during the wave formation, generating the pore. The calculated pressure and velocity of the wavefront were 20 MPa and 3500 m s−1, respectively. In conclusion, the SPH is suitable for fast previewing waveform interfaces, while the S-ALE method has the advantage of capturing fine waveform structures under various environmental conditions. [Display omitted] • Independent analyses of the impact welding process was carried out by the SPH and S-ALE methods. • The SPH and S-ALE methods revealed different reasons for describing the initial disturbance. • The two methods present their own advantages in facing different problems. • The S-ALE method can capture some defects generated by explosive welding in the atmosphere. [ABSTRACT FROM AUTHOR]

• Effects of hugoniot properties on the jet onset time, velocity and jetting side. • Jet initiates earlier at higher velocity if jetting side has higher shock parameter. • Jet initiates earlier at higher velocity if jetting side has higher sound velocity. • Jet initiates later and at lower velocity if jetting side has high density. • Effect of s-parameter and speed of sound is more profound than the density. Jets are the earliest ejected material from a hypervelocity impact event. Jets comprises of projectile and target's surface material and are ejected at velocities higher than escape velocity for Earth. Effects of Hugoniot properties of the projectile and target material on jet initiation are not well understood. We, therefore, developed a mathematical model that incorporated the limitations imposed by Hugoniot properties on jet initiation for hypervelocity impact. This model analysed the effects of changes in Hugoniot properties of the projectile and target on the jet onset time, jet onset velocity and jetting side (projectile or target). The results reveal that (i) jet initiates earlier and at higher velocity if the jetting side has higher value of shock parameter or higher velocity of sound in the material; (ii) jet initiates later and at lower velocity if jetting side has high density; (iii) effect of s-parameter and speed of sound in material is more profound than the effect of density. [ABSTRACT FROM AUTHOR]

Two types of intermediate layer (IML) were formed at the magnetic pulse welded Al/Fe joint interface. To clarify the formation mechanism of IML, the cooling process at the joint interface and formation process of IML were analyzed by the simulation and experiment. The simulation results showed that the crest zone of wavy interface underwent an extremely high temperature rise during the collision process, the high temperature leads to a local melting behavior at the joint interface. After the collision, a large temperature difference of 1000-1500K between joint interface and base metals caused a rapid cooling occurred at the interface with a cooling rate reached around 107~109K/s. Meanwhile, local melting zone (LMZ) changed into IML during a rapid solidification that proceeded from the outside and finished at the center area of LMZ. The morphology and composition percent of IML calculated by the numerical analysis was quantitatively in good agreement with experimental analysis results. A reasonable explanation has been made on the formation mechanism of IML at magnetic pulse welded Al/Fe joint interface. [ABSTRACT FROM AUTHOR]

A 1050-O aluminum plate and a low carbon steel (SPCC) plate were seam welded by magnetic pulse welding (MPW). Experimental and numerical analysis methods were combined to investigate the wavy interface formation and local melting phenomena at the Al/Fe joint interface. The metallographic observation results showed that Al/Fe joint interface presents a trigger-like wave shape with two types of intermediate layers intermittently formed at the interface. Collision angle between two plates and impact velocity of Al sheet driven by electromagnetic force was numerically solved by coupled mechanical and electromagnetic fields analysis. The metal jet emission behavior, wavy interface formation process and the temperature change during MPW collision process was analyzed by SPH method. The numerical results showed that the temperature near the interface exceeds the melting point of both Al and Fe at extremely high pressure, which leads to the formation of a local melting zone (LMZ) where Al and Fe were mixed in melted state. The numerically reproduced wavy interface morphology showed a good consistency with the experimentally observed results. [ABSTRACT FROM AUTHOR]

Collision welding is a joining technology that is based on the high-speed collision and the resulting plastic deformation of at least one joining partner. The ability to form a high-strength substance-to-substance bond between joining partners of dissimilar metals allows us to design a new generation of joints. However, the occurrence of process-specific phenomena during the high-speed collision, such as a so-called jet or wave formation in the interface, complicates the prediction of bond formation and the resulting bond properties. In this paper, the collision welding of aluminum and copper was investigated at the lower limits of the process. The experiments were performed on a model test rig and observed by high-speed imaging to determine the welding window, which was compared to the ones of similar material parings from former investigation. This allowed to deepen the understanding of the decisive mechanisms at the welding window boundaries. Furthermore, an optical and a scanning electron microscope with energy dispersive X-ray analysis were used to analyze the weld interface. The results showed the important and to date neglected role of the jet and/or the cloud of particles to extract energy from the collision zone, allowing bond formation without melting and intermetallic phases. [ABSTRACT FROM AUTHOR]

Introduced is a comprehensive numerical modeling framework that includes microstructure when simulating the laser impact welding (LIW) of metals to study the transient phenomena that occur during weld formation. Such transient phenomena include evolution of shear stresses, plastic strains, thermal response, and material jetting. Inhomogeneous microstructures for two dissimilar foils (aluminum 1100 and stainless steel 304) are first predicted using the Dynamic Kinetic Monte Carlo (KMC) method to simulate laser-based powder bed fusion (PBF-LB) additive manufacturing (AM). These microstructures are subsequently incorporated into an Eulerian finite element (FE) simulation of the LIW process, enabling prediction of grain elongations that result from the varying yield surfaces, stacking fault energies, and grain-boundary sliding effects. Trends in the predicted microstructure deformation patterns show strong agreement with those from experimental images in the literature. Compared to existing homogeneous models, the new framework with inhomogeneous AM microstructure reveals higher collision velocities at the weld interface, resulting in increased plastic strain rates, greater plastic heat dissipation, and increased material jetting with higher jet temperatures. The framework allows for new opportunities to study correlations between grain topography (as well as polycrystalline metal texture) and the transient process phenomena occurring at the impact weld interface.

Characteristic features of impact welding were investigated in a simple model system of colliding copper single crystals. Large grained sheets were used to approximate single crystal to single crystal impact. Optical microscopy and electron backscatter diffraction (EBSD) analyses showed multiple features of the weld interface, which include deformation twinning, adiabatic shear banding, localized melting, and dynamic recrystallization. To support the microstructural findings, numerical simulation based on smoothed particle hydrodynamics method was conducted to predict the local temperature, pressure, and strain distributions. The insights gathered here will aid in the design of materials combinations and systems for impact welding. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This paper mainly demonstrates an advanced type of the vaporizing foil actuator welding (VFAW) process between GPa-grade steel (TRIP1180) and aluminum alloy (AA5052-H32) without applying standoff. To secure a flying distance during the VFAW process, the preformed target sheet shaped like a circular indentation has been utilized. It is necessary to optimize process parameters integrated with geometrical design of the preform since the welding strength can be decreased beyond the optimum input energy in the standoff-free VFAW process. The welded surface was evaluated by SEM-EDS, XRD, EBDS, and TEM to analyze the welding mechanism and composition at the welding interface. The diffusion zone including the AlFe3 phase was observed at the welded interface which has high grain density due to the high-speed impact by increasing the welding strength, which leads to the perfect welding between the dissimilar materials.

Collision welding is a joining technology that is based on the high-speed collision and the resulting plastic deformation of at least one joining partner. The ability to form a high-strength substance-to-substance bond between joining partners of dissimilar metals allows us to design a new generation of joints. However, the occurrence of process-specific phenomena during the high-speed collision, such as a so-called jet or wave formation in the interface, complicates the prediction of bond formation and the resulting bond properties. In this paper, the collision welding of aluminum and copper was investigated at the lower limits of the process. The experiments were performed on a model test rig and observed by high-speed imaging to determine the welding window, which was compared to the ones of similar material parings from former investigation. This allowed to deepen the understanding of the decisive mechanisms at the welding window boundaries. Furthermore, an optical and a scanning electron microscope with energy dispersive X-ray analysis were used to analyze the weld interface. The results showed the important and to date neglected role of the jet and/or the cloud of particles to extract energy from the collision zone, allowing bond formation without melting and intermetallic phases.

The microstructure of aluminum‐steel compounds fabricated by magnetic pulse welding are investigated. Electron backscatter diffraction, energy dispersive x‐ray spectroscopy and scanning electron microscopy revealed the existence of several different phases along the weld seam. While one layer could be identified as aluminum solid solution, a very thin layer close to the steel side of the compound eluded detailed characterization by scanning electron microscopy techniques. Transmission electron microscopy and selected area electron diffraction investigations of this layer revealed a mix of nanocrystalline and amorphous parts. When ambient pressure was reduced to 1 mbar during welding, no interlayers were observed in the samples. This suggests that the interlayers are precipitates of the jet that is formed during conventional impact welding. [ABSTRACT FROM AUTHOR]

Vaporizing foil actuator welding (VFAW) was used for joining 2024-T3 and 7075-T6 aluminum alloy sheets, and the resulting joint microstructure was analyzed. 2024/7075 aluminum alloy pairs with suitable processing parameters can be prepared by using VFAW. Dynamic preform addresses the poor formability problem of target material and advantage of VFAW on dissimilar materials in some conditions. But with standoff sheet inserting in the flyer and target, 2024/7075 welded pairs gets the better weld strength, compared with flyer preformed method. The microstructure of the circular weld area of the welded joint showed a wave interface, in which a thin melt layer formed at the center and edge parts. The crystal grains near the bonding interface were remarkably elongated and refined. Therefore, the joining of the 2024/7075 pairs was facilitated through plastic forming and melting. [ABSTRACT FROM AUTHOR]

High velocity impact welding (HVIW) involves processes like the impact of metal structures and strong fluid-structure interactions with complex phenomena such as interfacial waves and jet generation. It is very difficult to model the HVIW process with typical physics well captured due to the large deformation and moving interfaces, while the associated mechanisms inherent in HVIW are also not well understood. In this paper, the HVIW process is simulated using a modified smoothed particle hydrodynamics (SPH) model, in which the kernel gradient correction is used to improve computational accuracy and an artificial stress term is used to ease stress instability during the welding process. The mechanisms in HVIW are investigated, and typical phenomena including the wavy interface, jet formation, interfacial temperature and pressure distribution are captured. It is demonstrated that with proper impact welding velocity and initial welding angle, the modified SPH method can well reproduce the morphology evolution of the welding interface from straight to wavy and further to wavy interface with vortex shedding. Based on comprehensive numerical data from SPH simulations, the weldability windows for the HVIW are obtained and are compared with experimental and theoretical results. Welding limits for HVIW are also discussed in detail. [ABSTRACT FROM AUTHOR]

This paper proposes a novel impact welding method named water-augmented vaporizing foil actuator welding, in which liquid water is introduced to alter the foil's vaporization process, thus altering the induced driving pressure and the final welding performance. A series of comparative experiments were performed to confirm the effectiveness of the process and to understand the underlying mechanism. The proposed process decreased the lower-bound required discharge energy by 58–69%, increased the peel strength by 30–60%, and significantly reduced the variability in the critical standoff distance at the lower bound of the weld window. In addition, the proposed process resolved the robustness issue by producing a much more regular and repeatable shape of the weld area compared to conventional VFAW. Furthermore, three potential mechanisms have been proposed to explain the process improvement: the added water 1) increased the electrical energy deposition, 2) introduced additional energy from the aluminum-water chemical reaction, and 3) provided a more efficient force-transfer medium. In summary, this paper introduces a novel impact welding method with much improved process capability and robustness compared to conventional vaporizing foil actuator welding. In addition, due to its simplicity, this method may be easily extended for other forms of impulse metalworking, which may stimulate a wide interest for the community of high velocity metalworking. [ABSTRACT FROM AUTHOR]

Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

Collision welding processes are accompanied by the ejection of a metal jet, a cloud of particles (CoP), or both phenomena, respectively. The purpose of this study is to investigate the formation, the characteristics as well as the influence of the CoP on weld formation. Impact welding experiments on three different setups in normal ambient atmosphere and under vacuum-like conditions are performed and monitored using a high-speed camera, accompanied by long-term exposures, recordings of the emission spectrum, and an evaluation of the CoP interaction with witness pins made of different materials. It was found that the CoP formed during the collision of the joining partners is compressed by the closing joining gap and particularly at small collision angles it can reach temperatures sufficient to melt the surfaces to be joined. This effect was proved using a tracer material that is detectable on the witness pins after welding. The formation of the CoP is reduced with increasing yield strength of the material and the escape of the CoP is hindered with increasing surface roughness. Both effects make welding with low-impact velocities difficult, whereas weld formation is facilitated using smooth surfaces and a reduced ambient pressure under vacuum-like conditions. Furthermore, the absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. This also enables an estimation of the temperature in the joining gap, which was found to be more than 5600 K under normal ambient pressure.

High-velocity impact welding is a kind of solid-state welding process that is one of the solutions for the joining of dissimilar materials that avoids intermetallics. Five main methods have been developed to date. These are gas gun welding (GGW), explosive welding (EXW), magnetic pulse welding (MPW), vaporizing foil actuator welding (VFAW), and laser impact welding (LIW). They all share a similar welding mechanism, but they also have different energy sources and different applications. This review mainly focuses on research related to the experimental setups of various welding methods, jet phenomenon, welding interface characteristics, and welding parameters. The introduction states the importance of high-velocity impact welding in the joining of dissimilar materials. The review of experimental setups provides the current situation and limitations of various welding processes. Jet phenomenon, welding interface characteristics, and welding parameters are all related to the welding mechanism. The conclusion and future work are summarized.

Explosive welding involves processes like the detonation of explosive, impact of metal structures and strong fluid-structure interaction, while the whole process of explosive welding has not been well modeled before. In this paper, a novel smoothed particle hydrodynamics (SPH) model is developed to simulate explosive welding. In the SPH model, a kernel gradient correction algorithm is used to achieve better computational accuracy. A density adapting technique which can effectively treat large density ratio is also proposed. The developed SPH model is firstly validated by simulating a benchmark problem of one-dimensional TNT detonation and an impact welding problem. The SPH model is then successfully applied to simulate the whole process of explosive welding. It is demonstrated that the presented SPH method can capture typical physics in explosive welding including explosion wave, welding surface morphology, jet flow and acceleration of the flyer plate. The welding angle obtained from the SPH simulation agrees well with that from a kinematic analysis. [ABSTRACT FROM AUTHOR]

Joining processes by high speed impacts such as explosion welding and electromagnetic pulse welding offer great advantages compared to conventional joining methods, especially fusion welding. However, a comprehensive understanding of the mechanisms and phenomena does not exist yet and thus process design largely involves empirical data. The established high speed joining technologies have in common that their observability for basic research is limited. It is the aim to provide valid and process independent data on necessary weld conditions which can be transferred to any impact welding process. A special test rig is used for basic research under reproducible conditions. The impact area is observed with high spatial and temporal resolution. Both the impact angle and the impact velocity can be adjusted individually and are constant during the process: The impact angle can be adjusted and measured with an accuracy of ± 0.05°, the normal impact velocity can be set in steps of 0.03 m/s up to 262 m/s. This publication focusses on the acquisition and discussion of a process window for aluminium specimens. Investigations on the joint strength will be carried out. It will be shown that the findings are in good accordance to analytical explanations and earlier works in literature. [ABSTRACT FROM AUTHOR]

Microsample tensile testing was conducted to evaluate the quality of impact welds created by vaporizing foil actuator welding. Tensile test samples with a gauge length of 0.6 mm were electro-discharge machined out of welds created between 1-mm-thick aluminum alloy type 6061 (AA6061) sheets and 6-mm-thick copper (Cu110) plates. Aluminum sheets were used as flyers, while copper plates acted as targets. Flyer sheets in T6 as well as T4 temper conditions were utilized to create welds. Some of the welds made with T4 temper flyers were heat treated to a T6 temper. It was found that the welds made with T4 temper flyers were slightly stronger (max. of 270 MPa) than those produced with T6 temper flyers. Generally, failure propagated in a brittle manner across the weld interface; however, elemental mapping reveals material transfer on either member of the welded system. This work proves the feasibility to apply microsample tensile testing to assess impact welding, even when conducted with flyer sheets of 1 mm or less, and provides insight that is complementary to other test methods. [ABSTRACT FROM AUTHOR]

This paper investigates the development and evolution of various interfacial features during high speed impact welding processes using numerical simulations. Arbitrary Lagrangian - Eulerian (ALE) and Eulerian computations are performed and compared with experimental features of welded joints. ALE method reveals poor mesh quality while the interface experiences severe shear deformation. Therefore, ALE simulations cannot accurately capture the morphological transition toward a wavy shape and cannot compute the successive jet formation from the interface. But, the Eulerian method becomes a good alternative to overcome those limitations of the ALE method. Computed interfacial features using Eulerian simulations are agreed well with the experimental observations of various interfacial phenomena. The actual kinematics along the whole length of the weld including wavy pattern, localized plastic deformation and shear strain orientation are clearly reproduced. Eulerian simulation also provides understanding for the potential cause of defects that corresponds to the experimental observations, in terms of morphology and site occurrence. [ABSTRACT FROM AUTHOR]

Using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS), we have investigated the microstructures of the bonding interface in Al/Fe clads fabricated by pulsed-magnetic-driven impact. A ∼10 nm thick interfacial layer of amorphous Al–Fe alloys has been observed to form at the interface of crystalline Al/Fe bimetallic system. Amorphization of polycrystalline Al takes place concurrently within the grain boundary triple junctions. It is suggested that amorphization at the bonding interface is initially driven by the collapse of the lattice during the high-strain-rate impact process and then followed by the interdiffusion reaction at temperature below the melting point. [ABSTRACT FROM AUTHOR]

Finite element simulations of high strain rate forming processes are of significant interest to industry, but are challenging due to the coupled physics and dynamic nature of the processes. For instance, in high velocity impact welding, extremely high plastic strain regions develop. Thus, a traditional pure Lagrangian analysis is not able to accurately model the process due to excessive element distortion near the contact zone. In this study, the Arbitrary Lagrangian-Eulerian method (ALE) was used to simulate an impact welding process for two Al6061-T6 plates. The ALE method was able to numerically predict the temperature at the interface as well as the necessary process parameters to achieve a wavy morphology at the interface of the materials which is assumed to be a characteristic of a quality impact weld. This allowed the prediction of a weldability window, including separate regions for melting and purely solid state welds. To validate the proposed numerical model and wavy morphology regions, experimental tests with a Vaporizing Foil Actuator Welding (VFAW) process were conducted including Photon Doppler Velocimetry (PDV) measurements of the impact velocity. The numerical results were in good agreement with the experimental tests including the temperature prediction and interfacial melting observed through microstructural analyses. [ABSTRACT FROM AUTHOR]

The objective of this study was to create impact welds between a Zr-based Bulk Metallic Glass (BMG) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications: impact welding of dissimilar materials being one of them. Its implementation for welding is termed as VFA Welding or VFAW. With 8 kJ input energy into an aluminum foil actuator, a 0.5 mm thick Cu110 alloy sheet was launched toward a BMG target resulting in an impact at a velocity of nearly 600 m/s. For this experiment, the welded interface was straight with a few BMG fragments embedded in the copper sheet in some regions. Hardness tests across the interface showed increase in strength on the copper side. Instrumented peel test resulted in failure in the parent copper sheet. A slower impact velocity during a separate experiment resulted in a weld, which had wavy regions along the interface and in peel failure again happened in the parent copper sheet. Some through-thickness cracks were observed in the BMG plate and there was some spall damage in the copper flyers. TEM electron diffraction on a sample, cut out from the wavy weld interface region using a focused ion beam, showed that devitrification of the BMG was completely avoided in this welding process. [ABSTRACT FROM AUTHOR]

High energy-rate metalworking has significant advantages, in that short time scales change the fundamental nature of the forming process and short duration impulses can be used with much lighter and more agile equipment because large static forces do not need to be resisted. Here, we propose the use of thin aluminum foils that are intentionally vaporized by a pulsed electric current, in order to create an intense mechanical impulse. The driving pressures can be a few GPa's, and have been used to launch sheet metal workpieces to velocities in excess of 1 km/s. Applications including impact welding, forming, embossing, shearing, and powder compaction have been demonstrated and will be presented in this paper. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel test rig to examine the principles of impact welding has been developed. [•] Numerical analyses of the impact conditions have been performed. [•] Microscopic investigation of the welded samples has been conducted. [•] The joints of the test rig and the real process are comparable. [•] The test rig can represent all stages of electromagnetic pulse welding. [Copyright &y& Elsevier]

Abstract: Three complementary impact welding technologies are described in this paper. They are explosive welding, magnetic pulse welding, and laser impact welding, which have been used to provide metallurgical bonds between both similar and dissimilar metal pairs. They share the physical principle that general impact-driven welding can be carried out by oblique impact but are used at different length scales from meters to sub-millimeter. The different length scales require different kinds of systems to drive the process, and the scales themselves can give different weld morphologies. Metallographic analysis on cross-sections shows a wavy interface morphology which is likely the result of an instability associated with jetting, which scours the surfaces clean during impact. The normalized period and amplitude of the undulations increase with increasing impact energy density. Microhardness testing results show the impact welded interface has a much greater hardness than the base metals. This can lead to weldments that have strengths equal to or greater than that of the weakest base material. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study and a numerical simulation of impact welding of single and double aluminium plates using a gas gun. The study shows the differences in the bonding quality of the two impacting flyer plates. In general the bonded area between the two flyer plates is less than that between the leading flyer and the target. The simulations show that the back surface of the leading plate flyer exhibits an increase in roughness after impacting onto the target. The numerical modelling in agreement with the experiment shows that the contact surface between two flyers at the time of the contact is not flat. [Copyright &y& Elsevier]

Abstract: The paper reports a study of high-speed oblique collision of non-flat metal plates with fixed target blocks. The main aim of the study was to improve the understanding of the industrial process of explosive welding. The results of tests with flat, V- and U-shaped flyer plates were presented. The effect of flyer shape on the area of bonding was examined and three-dimensional numerical modeling was used to identify and relate mechanical parameters, such as plastic strain and shear stress to the quality of welding. Effective plastic strain (PEEQ) and shear stress in the flyer plate were shown to be criteria in determining weld quality. The study has shown that the shape and amplitude of the waves in the interface between flyer plate and target block are dependent on the shape of the flyer plate. Both the numerical modelling and the experimental tests showed that the largest area of weld was produced with flat flyer plates. [Copyright &y& Elsevier]

A complete characterization of Cu-steel impact welded were made using photothermal radiometric (PTR) spectroscopy, scanning electron microscopy (SEM), microhardness, and X-ray diffraction (XRD) in order to study the changes in the structural, metallurgical, and thermal properties of Impact Welding Zone (IWZ) and the Impact Affected Zone (IAZ) after the impact welding. Three samples with different morphological interfaces were prepared for this analysis. According to the SEM analysis in etched samples, it was possible to determine that the IWZ is formed by the collapse of grains of Cu and steel (ferrite and pearlite) into the Cu-steel interface and the IAZ are formed in a region close to the IWZ with low grain damage. Microhardness Vickers test is able to detect the IAZ in all cases, but due to its contact character, it is not possible to obtain continuous hardness information across the Cu-steel interface. According to the XRD patterns after the impact welding process, no new phase was formed. Noncontact PTR amplitude and phase images are able to identify the IAZ and the IWZ. [ABSTRACT FROM AUTHOR]

Impact welding has been carried out using a projectile having a diameter of 11 and 5 mm at an impact velocity of 200 m/s or more. A large projectile regardless of the slenderness ratio, was welded to a stainless steel target. However, only a small projectile with a slenderness ratio less than 1.2 was welded to the same target. In order to examine the effect of the diameter of the projectile on the weldability, the temperature elevating process in two cylindrical projectiles subjected to a longitudinal impact was evaluated numerically using LS-DYNA. The numerical results indicated that since the deformation mechanism of a projectile with a slenderness ratio less than 1.0 is different of that of the projectile with the slenderness ratio more than 1.5, the small projectile with the slenderness ratio less than 1.0 can be welded to the target. [ABSTRACT FROM AUTHOR]